The general objectives of the MAC study are: (a) To better understand the natural history of HIV infection (b) To determine the risk factors, and appropriate interventions to limit the spread of the disease and to (c) Characterize any biologic or environmental factors which will alter the natural history of the disease (e.g, either limit or accelerate the morbidity and mortality associated with infection) in this population. The specific objectives of the MAC study are: (1) To continue to study the natural history of HIV infection among a population of homosexual men who have been enrolled in the MAC study since 1983, by specifically collecting demographic, sociocultural and behavioral selfreport data and statistically correlating this data with physical examination findings, HIV antibody status, immunologic and blood chemistry and hematology parameters and the presence of antibodies to other viruses. (2) To determine the rate of disease progression of infection by comparing laboratory findings (HLAtype, CBC, SMA12), immunologic parameters (by flow cytometry), physical examinations, neuropsychiatric evaluations and electromechanical findings (EEG, EKG, MRI, CAT, nerve conduction studies, radiographic findings) with behavioral events. (3) To determine the rate of seroconversion among sexually homosexual men as well as determine those behaviors which may alter the rate of seroconversion. (4) To determine the nature and extent of covirus infections among sexually active homosexual men.